A display device using a liquid crystal compound has been widely used for a display for a watch, a calculator, a word processor and so forth, wherein, in the invention, a term “liquid crystal compound” is used as a generic term for a compound showing a liquid crystal phase, and a compound showing no liquid crystal phase but being useful as a constituent of a liquid crystal composition. The display devices utilize a refractive index anisotropy, a dielectric anisotropy and so forth of the liquid crystal compound.
The liquid crystal phase includes a nematic phase, a smectic phase and a cholesteric phase. A product utilizing the nematic phase has most widely been used. Moreover, a display mode includes a dynamic scattering (DS) mode, a deformation of aligned phase (DAP) mode, a guest/host (GH) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a thin film transistor (TFT) mode, a vertical alignment (VA) mode, an in-plane switching (IPS) mode and a polymer sustained alignment (PSA) mode.
The liquid crystal compound used according to the display modes should show the liquid crystal phase in a wide temperature range centering on room temperature, be sufficiently stable under conditions in which the display device is used, and have sufficient characteristics for driving the display device. However, no single liquid crystal compound satisfying the conditions has been found out so far.
Therefore, a liquid crystal composition with required characteristics is actually prepared by mixing several kinds to several tens of kinds of liquid crystal compounds. The liquid crystal compositions are required to be stable to moisture, light, heat and air ordinarily present under the conditions in which the display device is used, to be also stable to an electric field and electromagnetic radiation, and to be furthermore chemically stable to a compound to be mixed. Moreover, the liquid crystal composition is needed to have suitable values of physical properties such as a refractive index anisotropy (Δn) value and a dielectric anisotropy (Δ∈) value depending on the display mode and a shape of the display device. Furthermore, each component in the liquid crystal composition importantly has a good solubility with each other.
In order to perform a good liquid crystal display, a cell thickness of a liquid crystal display device constituting the good liquid crystal display and a Δn value of a liquid crystal material to be used are preferably constant (E. Jakeman et al., Phys. Lett., 39A., p. 69 (1972)). Moreover, a response speed of the liquid crystal display device is inversely proportional to a square of thickness of a cell to be used. Therefore, a liquid crystal composition having a large Δn value should be available in order to manufacture a liquid crystal display device allowing a high speed response and also allowing application to displaying moving images and so forth. A variety of compounds have been developed as a liquid crystal single component having the large Δn value. In general, such a compound having the large Δn value is hard to use as a constituent of a liquid crystal composition having good electric characteristics because the compound has a highly conjugated molecular structure and tends to have a poor compatibility with other liquid crystal compounds. Furthermore, a high stability is required for a liquid crystal compound used as a constituent of a liquid crystal composition for a liquid crystal display device having a thin film transistor mode and so forth in which a high insulation (specific resistance) is required.
In order to solve the problems, a variety of compounds having a fluorovinyl group, a difluorovinyl group or an alkenyl group in a side chain have been synthesized as a compound that can be used for the liquid crystal display device. For example, compounds represented by formulas (S-1) to (S-3) are disclosed in Patent literature No. 1, a compound represented by formula (S-4) is disclosed in Patent literature No. 2, a compound represented by formula (S-5) is disclosed in Patent literature No. 3, a compound represented by formula (S-6) is disclosed in Patent literature No. 4, and a compound represented by formula (S-7) is disclosed in Patent literature No. 5.
However, the compounds represented by formulas (S-1), (S-2), (S-4) and (S-6) have only 46° C. in the widest temperature range of the nematic phase, and no nematic phase is developed in the compounds represented by formulas (S-3) and (S-7). No physical properties of the compound represented by formula (S-5) are disclosed. In any case, the known compounds cannot simultaneously satisfy a wide temperature range of the nematic phase, a low viscosity and a good compatibility with other liquid crystal compounds.
